1. Field of the Invention
The present invention relates to a directional beam communication system and a directional beam communication method for communicating with a mobile station located in a service area formed by a plurality of sectors using a directional beam which is transmitted by an array antenna including a plurality of antenna elements. Also the present invention relates to a base station and a controller which can used in the directional beam communication system and the directional beam communication method.
2. Description of the Related Art
A conventional mobile communication system applies a cellular system which covers a service area by arranging a plurality of base stations, each being associated to a cell.
The conventional mobile communication system employs a sector configuration in which a cell C is divided into a plurality of sectors S and a base station antenna is installed in each sector, so as to constitute the service area.
Generally, the conventional mobile communication system applies a three-sector configuration in which a cell C is divided into three sectors S as shown FIG. 1A, or a six-sector configuration in which a cell C is divided into six sectors S as shown FIG. 1B.
A technology for transmitting a directional beam by an adaptive array antenna is known as a technology to reduce interference on a mobile station caused by other mobile stations, in a mobile communication system.
The technology can reduce the interference power caused by another mobile station, by transmitting a directional beam using a plurality of antennas, and combining signals inputted to each of the antennas in accordance with proper weight values, so as to give directivity to signals to be transmitted.
For example, in the DS-CDMA (Direct Sequence Code Division Multiple Access) radio access system, a CAAAD (Coherent Adaptive Antenna Array Diversity) reception method using a pilot symbol in the uplink has been proposed.
An adaptive antenna array transmission method for compensating for a beam pattern formed in an uplink for amplitude and phase fluctuations of RF transmitter and receiver circuitries, and transmitting a directional beam having the compensated beam pattern in a downlink is known in the conventional mobile communication system.
The adaptive antenna array transmission method can reduce the interference power caused by another mobile station in the same sector and the interference power caused by another mobile station in another cell or sector, by transmitting the directional beam in the downlink.
Broadly speaking, there are two types of downlink channels in the conventional mobile communication station. One is a dedicated channel for transmitting individual information specific to each mobile station, and another is a common channel for transmitting control information common to all mobile stations in a sector.
FIG. 2A shows examples of beam patterns B1 and B2 of a beam for transmitting dedicated channels in the downlink. As shown in FIG. 2A, the base station BS transmits a directional beam having the beam pattern B1 to a mobile station MS#1 using an array antenna A, and the base station BS transmits a directional beam having the beam pattern B2 to a mobile station MS#2 using the array antenna A.
To be more specific, the base station BS narrows the beam of the dedicated channel for each mobile station MS#1 or MS#2 by the directional beam transmission, so that the interference power between the mobile stations MS#1 and MS#2 can be reduced.
On the other hand, FIG. 2B shows an example of beam pattern B3 of a beam for transmitting common channels in the downlink. As shown in FIG. 2B, the base station BS transmits a non-directional beam having the beam pattern B3 to the mobile stations MS#1 and MS#2 using the array antenna A.
Since the common channel for transmitting control information needs to be received by all mobile stations in the same sector, the common channel is transmitted by the non-directional beam as shown in FIG. 2B. The common channel can be transmitted by the sector antenna installed independent of the array antenna.
As shown in FIG. 3A, the base station which applies the adaptive antenna array transmission method is configured to have array antennas A1 to A3 for respective sectors S1 to S3, and to transmit the directional beam in accordance with weight values created for each sector independently.
Generally, the number of antenna elements which can be installed per cell is limited by a restriction of an installation condition of the base station BS.
For example, when a cell C includes six or more sectors (S1 to S6) as shown in FIG. 3B, the conventional adaptive antenna array transmission method arranges six antenna elements a1 or a2 at half wavelength intervals linearly as shown in FIGS. 4A and 4B, so as to form a directional beam with a beam width of 17° and to transmit the formed directional beam.
FIGS. 4A and 4B show only two adjacent sectors in the case where the number of antenna elements a1 or a2 of the array antenna A1 or A2 per sector S1 or S2 is six in the six-sector configuration. The horizontal beam width of the antenna element a1 is assumed to be 60°.
However, when the number of sectors S is large as described above, the number of antenna elements a1 or a2 of the array antenna A1 or A2 per sector S1 or S2 is small, so that the beam width of the directional beam transmitted by the array antenna A1 or A2 is increased. This results in a problem in that the interference suppression effect is reduced.
Especially, when a CDMA (Code Division Multiple Access) system is used as the radio access system, the base station generally spreads and modulates downlink signals using a scrambling code specific for identifying each sector and a channelization code for identifying each channel within each sector.
At this time, the channelization code generally employs an orthogonal code, and the orthogonality between channels using a same scrambling code can be maintained. Accordingly, the interference caused between the channels can be controlled to be small.
Herein, in the case of the three-sector configuration, since the number of antenna elements of the array antenna per sector is comparatively large, and the directional beam transmitted by the array antenna has a small beam width, so that a large interference suppression effect can be obtained.
The number of channelization codes is N if the spreading factor of the channelization code is N. Herein, in the case where N or more channels are accommodated in a sector, there is a shortage of channelization codes, and it is necessary to communicate using a plurality of scrambling codes within a same sector for identifying channels.
However, when the plurality of scrambling codes are used within a same sector, interference is caused between channels to which different scrambling codes are assigned, and this results in a problem in that the system capacity is reduced.
On the other hand, in the case of the six-sector configuration, the area per sector is half of that in the case of the three-sector configuration, so that the number of mobile stations accommodated per sector is half. Therefore, there is no shortage of channelization codes.
However, since the number of antenna elements of the array antenna per sector is reduced as previously described, the beam width of the directional beam transmitted by the array antenna is increased, thus resulting in a reduced interference suppression effect.